Combine power feed and data link via cable for remote peripherals
In many communication systems, remote devices are connected to main units through cable assemblies. The data and control commands are exchanged between each remote device and the main units. Very often these devices are placed in locations with limited power access. Camera modules used in automotive safety (backup cameras) and security surveillance applications are good examples of remote devices. The most convenient way to deliver power to them is through the cable assembly. This article outlines the different methods to send power through a cable assembly, and discusses an implementation to combine both power and data over a shared transmission medium.
Power feed with copper wires
Many transmission standards have incorporated the power distribution capability in the transmission medium. Universal Serial Bus (USB) is an example in which 5V is distributed through a pair of copper wires in the USB cable assembly. It is widely used for providing power to remote peripherals in PC and consumer applications. Using copper wires is the most cost-effective and robust method for delivering power to remote devices.
The figure below illustrates the use of a dedicated pair of copper wires to provide power to a remote camera, while the captured image is serialized by the DS90UB901Q and sent through a separate twisted pair to the DS90UB902Q deserializer at the main processing unit. One pair of wires is used to transport the video data and control information through the FPD (Flat Panel Display Link)-Link III serializer and deserialzer (SerDes), while the other pair is used to carry power.
Power feed with coaxial cable
Sharing a single transmission medium between signal and power eliminates the need for a second pair of wires. Often a second pair may be deemed inconvenient, or cannot be used for certain applications. The use of power and signal over a shared medium is not a new concept and has been widely used in television signal amplifiers located in roof-top antennas (see figure below).
Power is delivered by the same coaxial cable that carries the antenna signal to the TV receiver located in the house. As illustrated in the figure, DC power is injected into the coaxial cable through an inductor L1 at the source, and extracted at the destination with inductor L2. The power return path runs through the braided shield of the coaxial cable. As long as L1 and L2 provide sufficiently high impedance, they appear as an open circuit to the high frequency TV signals, and do not affect signal fidelity.
Power feed with shielded twisted pair cable
The concept of sharing power and signal over one transmission medium can be applied to high-speed serial links such as TI’s FPD-Link II/III and Channel Link II/III SerDes families. The figure below shows a remote camera implemented with the DS90UB901Q serializer and DS90UB902Q deserializer, with power and data transmitted over one shielded twisted pair cable.
The power feed is applied equally to the two wires of the differential pair through a pair of inductors at L3. Power is extracted equally from the two wires at the destination through L4. Any switching noise from the power supply is applied equally at the two wires and they appear as common-mode signals. The differential receiver of the deserializer is capable of rejecting common-mode signals and minimizing the effect from the power supply’s switching noise. The DC return path is carried by the cable’s outside braided shield.
In applications where the braided shield cannot be used as the DC return path, power and return can be applied to the two wires of the differential pair. The following figure illustrates the arrangement with a differential power feed. Any switching noise from the power supply will appear differentially and superimposed onto the high-speed differential signals. In the presence of a large amount of supply noise, this approach is more vulnerable to performance degradation due to the potential for signal-to-noise erosion.
Requirements of power feed network
The figure below illustrates a conceptual design for power and serial data over a shared data pair. The power feed inductors L3 and L4 are shunting components on both ends of the cable. They are designed to present sufficiently high AC impedance (ZDIFF) so they do not materially lower the cable impedance, and do not adversely affect its ability to transmit the differential signal. The inductors must also be able to withstand the DC current load without causing magnetic saturation, whereby the inductance collapses and significantly lowers the inductor’s impedance and negatively impacts the cable’s impedance.
L3 can be built with two inductors in separate packages. However, it is advantageous to use a pair of mutually coupled inductors with both coils wound on a common ferrite core. In the above figure, L3 and L4 are differential chokes with a center tap where the DC power is injected or extracted. The DC currents through the two windings of L3 create magnetic fluxes of opposite polarity. The magnetic fluxes cancel each other and avoid magnetic saturation. The use of mutually coupled inductors helps to reduce the physical size of the ferrite core.
Impedance characteristics and performance trade offs
The inductor value is aimed at providing high AC impedance over the operating frequency range of the differential signals. The figure below shows an impedance plot with different inductor values. An inductance of about 10 µH is capable of achieving high shunting impedance for the FPD-Link II and Channel Link II SerDes, with serial data rates of up to 1.5 Gb/s. The table after the figure shows the serial data rates of FPD-Link II and III SerDes.
FPD-Link III and Channel-Link III SerDes support bidirectional data with high-speed serial bit streams at the forward direction (forward channel), and a low-speed control data at the reverse direction (back-channel). An inductor value of about 1 to 10 µH is used to support the gigabit forward channel, and a larger inductor of about 150 µH is needed to support the low speed back-channel of a few Mbps. The following figure illustrates the impedance characteristic of cascaded inductors.
The design of a power feed inductor involves optimization of stray parasitics to achieve a wide bandwidth inductor and very often involves trade-offs in physical size and saturation current. The figure below illustrates the characteristics of differential chokes L3 and L4 used for the power feed. The differential choke is capable of providing a high impedance to support 1.5Gbps data transmissions for the FPD-Link III DS90UB901Q/902Q SerDes, and delivers a DC current of 0.5A without magnetic saturation.
Impedance characteristics of differential choke KA4909-AL from CoilCraft
Along the path of the power feed, there are DC resistances in the power feed inductors (L3 and L4), as well as internal resistance in the cable. To allow for the small voltage drop caused by the DC resistances, a 5V supply is used for VFEED to provide a 2.5W (5V x 0.5A) power feed to the remote camera. Regulators at the sink-side are used to convert 5V into 3.3V and 1.8V for the DS90UB901Q serializer, and for powering the image sensor. A 12V supply for VFEED provides a 6W solution (12V x 0.5A). Down converters are used at the sink-side to convert 12V into lower regulated voltages. Some amount of filtering may be necessary at the input-side of the down-converters to avoid excessive switching noise from going into the two wires, which would appear as common-mode noise.
Performance trade-offs
The viability of sharing power with the data pair wires lies with the challenge of minimizing any performance impact to the data carrying differential signals. The differential choke L3 at the source and L4 at the sink are connected in parallel to the differential pair and will lower the cable’s impedance. A cable with good return loss and properly designed power feed network (with sufficiently high impedance) is able to withstand the potential impedance degradation, and achieves a combined return loss of about 20dB.
The figure below illustrates a comparison of data eye patterns at 1.5 Gbps with and without the power feed networks. The data eye diagram shows only a slight increase in jitter, proving the cable with power feed is more than capable of supporting 1.5 Gbps data transmissions for megapixel camera applications.
Data eye diagrams after 2m cable – with and without power feed
An implementation for 2.5W remote camera application
In many camera applications, 2.5W is capable of powering the camera sensor, the serializer, and other supporting circuitries. The following figure illustrates a 2.5W implementation for a remote camera application.
The serialized video image from the camera sensor and the embedded bidirectional I2C control data are transmitted through a single shielded twisted pair cable, along with the DC power feed. At the host unit where the deserializer resides, the local 12V power supply is converted into 5V with the LM22671 switcher. The 5V is then fed into the two wires of the cable with the use of a power feed inductor L3.
At the remote side, the DC voltage is extracted from the center tap of the power feed inductor L4. The LP38693 adjustable LDO’s provide regulated 1.8V and 3.3V supply rails. The CoilCraft KA4909 is used for the power feed networks L3 and L4. It is designed to provide high differential impedance within the frequency range of 1 to 1,000 MHz at a DC saturation current of greater than 0.5A. Higher power can be achieved by raising the VFEED voltage, or raising the DC saturation current of the power feed network with the trade-off of a larger magnetic core.
Conclusion
There are multiple ways to deliver power to a remote device that has limited access to a power source. Power distribution with copper wires remains the lowest cost and most robust solution. In applications that have the need to reduce the size and weight of an extra pair of copper wires, it is possible to combine power distribution and high-speed data transmission by sharing the same shielded twisted pair cable. A power feed network will be necessary at both ends of the cable for power injection and extraction. By properly designing a power feed network, and managing the power supply’s switching noise, the performance degradation to the differential data transmitted through the shared cable can be minimized.
Tsun-kit Chin and Dac Tran are applications engineers at Texas Instruments.
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